With respect to steel structures such as a bridge or the like, an appropriate maintenance work is required for these structures so that aged deterioration in load bearing ability and durability of such structures may not reduce the safety of them. Especially, under some circumstances where external forces are repeatedly applied to steel materials of the steel structure, for example, in a bridge to which a load is repeatedly applied along with the passage of vehicles, cracks may occur in places such as welded portions in which stress may easily concentrate, and such cracks may cause an event reducing the safety, such as fracture of the steel material, and accordingly, there is a strong demand to detect early the cracks.
The major conventional ways to detect a crack caused in a steel material of a steel structure may include a magnaflux method, a penetrant testing method and an ultrasonic testing method.
JP 2001-21539 A (Patent Literature 1) discloses an example of the conventional magnaflux method. JP 2003-344303 A (Patent Literature 2) discloses an example of the penetrant testing method. In addition, JP 2005-345217 A (Patent Literature 3) discloses an example of the ultrasonic testing method.